This invention relates to controlling porosity in metal cast within a particulate mold by a lost foam process. More particularly, this invention relates to a lost foam casting process that employs a chill member to reduce or eliminate porosity in a region of a product casting.
In a lost foam metal casting process, a refractory coated vaporizable polymeric pattern is embedded in a mold formed of unbonded refractory particles, i.e., sand, and is decomposed and replaced by molten metal. The pattern is preferably embedded while fluidizing the refractory particles. As the molten metal decomposes the pattern, a substantial volume of vapors vent into the sand mold, thereby allowing the melt to flow in and completely duplicate the pattern. Thereafter, the metal cools and solidifies to form a product casting, whereupon pores tend to form in the metal as a result of precipitation of gases, such as hydrogen, and metal contraction. This porosity undesirably reduces mechanical properties.
Chill members have been employed to locally reduce porosity in metal cast by a process of the type wherein a rigid mold defines a casting cavity, for example, green sand casting. The chill forms a portion of the cavity surface, securely held by the surrounding rigid mold, and accelerates cooling of adjacent metal to quickly solidify a region of the casting before outgassing or shrinkage can form pores. However, chill members have not heretofore been successfully employed for lost foam casting within a nonrigid, cavityless mold. This is attributed in part to the difficulty in effectively arranging a chill and the pattern while forming the mold by fluidization. It has been found that the chill must be positioned in the mold immediately adjacent the pattern. Space between the chill and the pattern fills with melt during casting to prevent pattern surface duplication. Sand or a refractory coating between the members thermally insulates the chill to reduce its cooling effect. It is particularly difficult to exclude fluidized sand from infiltrating between separate members. Furthermore, it has been found that mere separate positioning of a pattern and a chill in the mold, even with great care, does not necessarily achieve the desired local reduction in porosity, perhaps because the position of the chill is not maintained during casting.
Therefore, it is an object of this invention to provide a lost foam metal casting process that employs a chill member to reduce porosity in a region of a product casting, which process comprises assembling a vaporizable pattern and a chill in a fixed arrangement prior to inserting into a fluidized particulate bed to form a mold. This permits the chill and the pattern to be arranged without the inconvenience of the mold forming operations. Thus, the chill may be reliably and effectively positioned adjacent a predetermined pattern surface without sand or space therebetween. Furthermore, the fixed arrangement is maintained not only as the mold is formed, but also during casting until melt has substantially replaced the adjacent pattern, so that thereafter the chill is located effectively to accelerate cooling and solidification and thereby reduce porosity. Premold pattern-chill assembly as provided by this invention is particularly useful for rapidly and conveniently preparing a plurality of molds in a mass production operation.